1. Field of the Invention
The present invention relates to an illumination system. More particularly, the present invention relates to an illumination system using a coherent light source.
2. Description of Related Art
Referring to FIG. 1, a conventional laser projection apparatus 100 including an illumination system 110, a digital micro-mirror device (DMD) 120, and a projection lens 130. The illumination system 110 includes a laser light source 112, a plurality of lenses 114, and a light integration rod 116. A laser beam 113 emitted from the laser light source 112 is expanded by the lens 114 and uniformed by the light integration rod 116 to become an illumination beam 113a. After being projected onto the DMD, the illumination beam 113a is converted by the DMD into an image beam 113b. The image beam 113b is projected onto a screen (not shown) through the projection lens 130, thus forming a display image.
FIG. 2 shows a speckle pattern projected on the screen by the laser projection apparatus in FIG. 1. Referring to FIGS. 1 and 2, due to the high coherence of the laser beam 113, after the laser beam 113 passes through an optical component (such as a lens, a reflector) having a slightly uneven surface in the laser projection apparatus 100, a speckle pattern formed by the laser beam 113 is generated on the screen due to the interference phenomenon of the laser beam 113. The speckle pattern is an irregular noise pattern. The speckle phenomenon causes a non-uniform brightness of the display image, thus degrading the optical quality of the laser projection apparatus 100.